1. Technical Field
The present invention relates to an image processing controller that performs screen processing on input image data for reducing the number of tones thereof to three or more inclusive of halftone and generates output image data. The output image data is supplied to a printing unit. In addition, the invention relates to a printing apparatus that forms an output image.
2. Related Art
FIG. 13 is a block diagram that schematically illustrates an example of the configuration of a laser printer 910 according to related art. The laser printer 910 includes a screen processing unit 911, a PWM (Pulse Width Modulation) unit 912, and a print engine 913. The laser printer 910 outputs a print image 931 corresponding to input image data 920. The input image data 920 contains a bitmap image expressed in 256 grayscale levels. The input image data 920 has an 8-bit tone value (i.e., an integer ranging from 0 inclusive to 255 inclusive) in each of a plurality of pixels 921. The screen processing unit 911 performs screen processing, which is tone-reduction processing performed for each of the pixels 921 of the input image data 920 for reducing the 8-bit tone value in 256 grayscale levels into a 4-bit tone value in 16 grayscale levels. The Image data in 16 grayscale levels is outputted to the PWM unit 912. The PWM unit 912 converts the image data outputted from the screen processing unit 911, which has the 4-bit tone value (i.e., an integer ranging from 0 inclusive to 15 inclusive) in each of the pixels 921, into a pulse signal that is suited for laser-beam dot formation. The PWM unit 912 outputs the pulse signal to the print engine 913. In accordance with the pulse signal, the print engine 913 forms toner dots on a print target medium 930 to output the print image 931.
A printing apparatus that can perform printing in a “toner save” mode is known in the art. The toner save mode is an operation mode for reducing the consumption of toner, which is an example of a recording material. The following schemes are known as methods for offering such a toner save mode: (1) a method of reducing the power of laser beam emission of the print engine 913, (2) a method of decimating dots to form a specific pattern of dot arrangement at the screen processing unit 911 (refer to FIG. 1(b) in JP-A-2001-230930), and (3) a method of reducing the tone value of image data outputted from the screen processing unit 911.
FIG. 14A is a diagram that schematically illustrates an example of the above method (2) of decimating dots. For example, it is assumed that output image data 941 having a tone value of 15 (which is the maximum value in 16 grayscale levels) in each of a plurality of pixels is generated in a normal mode. The normal mode is a mode for forming the print image 931 by using the normal amount of toner. In a toner save mode, that is, a mode for forming the print image 931 while saving toner, output image data 942 having a predetermined pattern due to dot decimation at some pixels is generated.
FIG. 14B is a diagram that schematically illustrates an example of the above method (3) of reducing the tone value of image data outputted from the screen processing unit 911. For example, it is assumed that output image data 943 having a tone value of 15 (which is the maximum value in 16 grayscale levels) in each of a plurality of pixels is generated in the normal mode. In the toner save mode, output image data 944 having a reduced tone value of 7, which is a halftone value in 16 grayscale levels, in each of the plurality of pixels is generated.
An image processing method that can save toner without performing any additional processing that is unique to the saving of toner concurrently with color processing is disclosed in JP-A-2003-295701. In the image processing method disclosed therein, a toner saving rate has been pre-applied to a gamma table that is used during color conversion. That is, the tone value of image data before being inputted into a screen processing unit has been reduced in advance. By this means, the tone value of image data outputted from the screen processing unit is reduced.
The above method (1) of reducing the power of laser beam emission has a disadvantage in that it is necessary to provide some means for adjusting the power of laser beam emission. Therefore, a laser printer that is not provided with any means for adjusting the power of laser beam emission cannot offer the toner save mode. The above method (2) of decimating dots has a disadvantage in that an unintended pattern or jaggies may be generated depending on a dot decimation pattern. The above method (3) of reducing the tone value of output image data has a disadvantage in that, as illustrated in FIG. 14C, an output can be expressed in the toner save mode in eight grayscale levels only, which correspond to tone values ranging from 0 inclusive to 7 inclusive, although the output can be expressed in the normal mode in 16 grayscale levels, which correspond to all tone values ranging from 0 inclusive to 15 inclusive. In other words, the number of grayscale levels available for reproduction of the output image data in the toner save mode is eight only.
An image processing apparatus disclosed in JP-A-2001-230930 includes a TRC (Tone Reproduction Curve) correction unit. The image processing apparatus disclosed therein converts an 8-bit color component signal for each of cyan (C), magenta (M), yellow (Y), and black (K) into a 1-bit color component signal for each of C, M, Y, and K on a pixel-by-pixel basis at the TRC correction unit. That is, the image processing apparatus disclosed therein has a problem in that, since there are two tones only for a color-separated signal after the TRC correction processing, halftone information will be lost. For this reason, a print image can be expressed in two grayscale levels only.